Most inorganic and some organic chemical compounds, when in a molten state or when dissolved in water or other liquids, become ionized; that is, their molecules become dissociated into positively and negatively charged components, which have the property of conducting an electric current. If a pair of electrodes is placed in a solution of an electrolyte, or an ionizable compound, and a source of direct current is connected between them, the positive ions in the solution move toward the negative electrode and the negative ions toward the positive. On reaching the electrodes, the ions may gain or lose electrons and be transformed into neutral atoms or molecules, the nature of the electrode reactions depending on the potential difference, or voltage, applied.
The action of a current on an electrolyte can be understood from a simple example. If the salt copper sulfate is dissolved in water, it dissociates into positive copper ions and negative sulfate ions. When a potential difference is applied to the electrodes, the copper ions move to the negative electrode, are discharged, and are deposited on the electrode as metallic copper. The sulfate ions, when discharged at the positive electrode, are unstable and combine with the water of the solution to form sulfuric acid and oxygen. Such decomposition caused by an electric current is called electrolysis.
Electrolysis has industrial applicability in a process known as electroplating. Electroplating is an electrochemical process for depositing a thin layer of metal on, usually, a metallic base. Objects are electroplated to prevent corrosion, to obtain a hard surface or attractive finish, to purify metals (as in the electrorefining of copper), to separate metals for quantitative analysis, or, as in electrotyping, to reproduce a form from a mold. Cadmium, chromium, copper, gold, nickel, silver, and tin are the metals most often used in plating. Typical products of electroplating are silver-plated tableware, chromium-plated automobile accessories, and tin-plated food containers.
In the process of electroplating, the object to be coated is placed in a solution, called a bath, of a salt of the coating metal, and is connected to the negative terminal of an external source of electricity. Another conductor, often composed of the coating metal, is connected to the positive terminal of the electric source. A steady direct current of low voltage, usually from 1 to 6 V, is required for the process. When the current is passed through the solution, atoms of the plating metal deposit out of the solution onto the cathode, the negative electrode. These atoms are replaced in the bath by atoms from the anode (positive electrode), if it is composed of the same metal, as with copper and silver. Otherwise they are replaced by periodic additions of the salt to the bath, as with gold and chromium. In either case equilibrium between the metal coming out of solution and the metal entering is maintained until the object is plated.
Recently recognized applications of electroplating relate to the electroplating of a semiconductor wafer. The electroplated metal is used to provide the interconnect layers on the semiconductor wafer during the fabrication of integrated circuit devices. Due to the minute size of the integrated circuit devices, the electroplating process must be extremely accurate and controllable. To ensure a strong and close bond between the wafer to be plated and the plating material, the wafer is cleaned thoroughly using a chemical process, or by making it the anode in a cleaning bath for an instant. To control irregularities in the depth of the plated layer, and to ensure that the grain at the surface of the plated layers is of good quality, the current density (amperes per square foot of cathode surface) and temperature of the wafer must be carefully controlled.
The present inventors have recognized this need for controlling irregularities in the depth of the plated layer across the surface of the wafer. The present invention is directed, among other things, to a solution to this problem.